The present invention relates generally to ablation catheter systems that use electromagnetic energy in the microwave frequencies to ablate internal bodily materials. More particularly, a variety of tuning arrangements are disclosed for impedance matching the power supply and catheter microwave transmission line components in order to minimize reflected power and maximize catheter to tissue coupling.
Catheter ablation has recently become an important therapy for certain cardiac arrhythmias. Radio frequency (RF) energy is presently accepted as the preferred ablating energy source. Accordingly, a variety of RF catheters and power supplies are currently available to electrophysiologists. Radio frequency energy has several limitations including the rapid dissipation of energy in surface tissues resulting in shallow "burns" and failure to access deeper arrhythmic tissues. Another limitation is the risk of clot formation on the energy emitting electrodes. Such clots have an associated danger of causing potentially lethal strokes in the event that a clot is dislodged from the catheter. A second common ablation approach is the use of high voltage, direct current defibrillator discharges. Direct current ablation has several drawbacks including the need for general anesthesia and explosive discharges that can cause debris or even rupture certain cardiac organs. For these and other reasons, significant attention has been given recently to alternative ablative energy sources.
Microwave frequency energy has long been recognized as an effective energy source for heating biological tissues and has seen use in such hyperthermia applications as cancer treatment and preheating of blood prior to infusions. Accordingly, in view of the drawbacks of the traditional catheter ablation techniques, there has recently been an interest in using microwave energy as an ablation energy source. The advantage of microwave energy is that it is much easier to control and safer than direct current applications and it is capable of generating substantially larger lesions than RF catheters, which greatly simplifies the actual ablation procedures.
In U.S. Pat. No. 4,641,649, Walinsky et al. disclose a medical procedure for the treatment of tachycardia and cardiac disrhythmia which uses microwave frequency electromagnetic energy to ablate selected cardiac tissue. The microwave energy is transmitted over a coaxial transmission line having an antenna at its distal end. A similar procedure is disclosed in the article by Langberg et al. entitled "Catheter Ablation of the Atrioventricular Junction Using a Helical Microwave Antenna: A Novel Means of Coupling Energy to the Endocardium," PACE, pp. 2105-2113 Vol. 14 (1991). As suggested in the title, the Langberg et al. article proposes the use of a helical microwave antenna at the distal end of the catheter in order to improve the catheter's power delivery characteristics. In U.S. Pat. Nos. 4,945,912, and 5,246,438, Langberg details particular helical antenna designs to be used for cardiac tissue ablation. In the later patent, Langberg recognizes the importance of adjusting the catheter impedance for particular ablation conditions. However, he proposes setting a particular presumed optimal impedance during fabrication. However, this design is not real-time tunable to compensate for the time variation of the impedance over the course of an ablation procedure.